Subtidal Sea Level Research Articles

On the relative importance of the remote and local wind effects to the subtidal variability in a coastal plain estuary

A set of month‐long sea level and current measurements is used to examine the subtidal variability in the Delaware estuary and assess the relative importance of remote and local wind effects on the observed subtidal variability. The evidence indicates that the remote wind effect, through the impingement of coastal sea level at the mouth of the estuary, is more important than the local wind effect in producing the subtidal sea level fluctuations in the interior of the estuary. On the other hand, the local wind effect dominates over the remote wind effect in producing the observed subtidal current fluctuations at the mooring site. It appears that the remote wind effect is important to the sectionally averaged subtidal transport into or out of the estuary. However, the local wind effect may be more important than the remote wind effect in producing the subtidal currents at any given point along the estuary's cross section. The spatial structure associated with the local wind‐induced circulation is very important to the long term transport and distribution of waterborne material.

Numerical Simulation of the Exchange Process within a Shallow Bar-Built Estuary

Great South Bay is the largest of a series of interconnected bar-built estuaries on the south shore of Long Island, New York. The depth-averaged barotropic motions in the bay were simulated by using a finite element two-dimensional numerical model. The barotropic motions were driven with astronomical tides, subtidal coastal sea level fluctuations induced by longshore wind stress over the adjacent continental shelf, and local wind stress over the surface of the bay. There was vigorous exchange at tidal frequencies between the western part of Great South Bay and the surrounding waters, but the tidal exchange was heavily damped in the eastern part of the bay. At subtidal frequencies the volume exchange did not exhibit significant attenuation in the interior of the bay. In the eastern part of Great South Bay, the magnitude of the subtidal volume exchange could exceed that of the tidal exchange. The principal mode of subtidal volume exchange was found to be associated with subtidal sea level fluctuations along the coast, which characteristically caused a filling or emptying of the system from all open boundaries of Great South Bay.

On the Contribution of Subtidal Volume Fluxes to Algal Blooms in Long Island Estuaries

A bloom of a new picoplankter species Aureococcus anophagefferens occurred in several estuaries in the northeast coast of the U.S.A. in 1985. The phenomenon is believed to be the result of a complex chain of physical-chemical-biological factors. Based on 9 years (1980-88) of tidal data taken at three oceanic locations (Nantucket, Montauk, Atlantic City) and four impacted estuaries on Long Island (Peconic, Shinnecock, Moriches and Great South Bays) it was found that the variance of the sea level fluctuations at subtidal frequencies was much higher than that of the tidal frequencies and highly dependent upon the season. These subtidal variances reached an absolute minimum in the spring of 1985, corresponding to the lowest subtidal volume exchange between shelf waters and the estuaries. Subtidal flushing times were computed taking into account an estimated recirculation parameter. These times were at an absolute high in the spring of 1985; increases of 16, 24 and 12% were determined for Great South Bay, Moriches and Shinnecock respectively. Along with reduced freshwater inflows due to the existing drought, the increased flushing times may have supported a longer permanence of available inorganic nutrient compounds contributing to a favourable environment for the ecological dominance of Aureococcus. The low subtidal sea level fluctuations of 1985 are related to a minimum energy content of the regional wind stress, particularly the alongshore (East-West) component which is responsible for the Ekman pumping into and out of the estuaries.

Subtidal sea level fluctuations in a large fjord system

A cross‐spectral analysis is made of subtidal (0.01–0.5 cpd) sea level (SSL), atmospheric pressure and wind fluctuations observed during 1966–1988 in the southern reaches of the fjord system formed by the Straits of Juan de Fuca/Georgia and Puget Sound. It shows that observed SSL can be largely explained by quasi‐steady inverse barometer compensation to atmospheric pressure and the propagation of adjusted sea level fluctuations from the nearby continental shelf, with some local weather forcing in summer. Variations in coherence, amplitudes and relative phase between seasons, locations and frequencies support previous work that both barotropic and baroclinic processes have important influences on SSL within the fjord system.

Sea Level Variability in Long Island Sound

Sea level variability in Long Island Sound is examined at both tidal and subtidal frequencies over a 1-yr period. The sound is found to be decoupled effectively from the lower Hudson Estuary at tidal frequencies. The predominantly semidiurnal tides in the sound are forced by the oceanic tides transmitted from the mouth. There is a near fourfold amplification of the semi-diurnal tides within the sound due to resonance. Diurnal tides are much weaker in the sound, and there is also no evidence of significant amplification in the interior. At subtidal frequencies, the pressure-adjusted sea level in the interior of the sound is forced by a combination of co-oscillation with coastal sea level at the mouth and direct setup induced by local wind forcing over the surface of the sound. Because the longitudinal axis of Long Island Sound is roughly aligned with the open coast from Montauk Point to Sandy Hook, these two mechanisms work in concert to produce larger subtidal sea level fluctuations in the western sound relative to those in the eastern sound. A linearized, frequency-dependent analytical model is developed to aid the interpretation of field observations.

The current and sea level variability in the Chesapeake and Delaware Canal

Current and sea level observations in the Chesapeake and Delaware (C&D) Canal are used to examine the frequency‐dependent spatial distributions of tidal and subtidal variability along the canal. To first order of approximation, the C&D Canal is shown to be a linear system. A linearized, frequency‐dependent analytical model is developed to examine details in the tidal and subtidal variability along the canal, and results agree well with observations. The principal semidiurnal tides (M2) at the two ends of the canal are roughly in phase but differ substantially in amplitude. The amplitude of the M2 tide increases monotonically from the Chesapeake end of the canal toward the Delaware end. The amplitude of the M2 current is of the order 60 cm s−1, and it is higher in the western part of the canal than in the eastern part of the canal. The amplitudes of the principal diurnal tides (K1) at the two ends of the canal are much smaller than those for M2, but the K1 tides at the ends are nearly 180° out of phase with each other. The amplitude of the K1 tide decreases toward the interior of the canal from both ends. Relatively strong K1 current of the order 20 cm s−1 exists in the canal, and its amplitude shows only slight variation along the canal. There are substantial differences in the subtidal sea level fluctuations along the canal, resulting in strong subtidal currents which may exceed 70 cm s−1. The importance of the C&D Canal on the upper Delaware Bay is assessed by comparing the volume flux through the canal relative to that in the upper Delaware. It is shown that the influence of the canal on the Delaware depends strongly on the time scales involved. It appears that the C&D Canal is not important to the overall tidal response in upper Delaware Bay, but the subtidal volume flux through the canal must be included for a proper assessment of the dynamics of upper Delaware Bay at subtidal frequencies.

Subtidal volume exchange through the Chesapeake and Delaware Canal

The Chesapeake and Delaware Canal is a man‐made waterway connecting the head of the Chesapeake Bay and the upper reaches of the Delaware estuary. We found significant subtidal variability along the canal during a 1‐month survey in 1975. There were significant differences between subtidal sea level fluctuations at the two ends of the canal, reflecting the different response characteristics of the two adjoining estuaries to atmospheric forcing. The resulting hydraulic head along the canal forced energetic subtidal volume flux through the canal which was of the same order of magnitude as the Delaware River discharge during the spring freshet period.

Tidal and Subtidal Variability in Delaware's Inland Bays

Sea level observations made during a three-month period in late 1984 at four stations in and near Delaware's inland bays are used for the examination of tidal and subtidal variability in the bays. Five tidal Constituents are found to be active in the bays, With M2 being the most important one. The inland bays, with their narrow inlets and shallow depths, behave like low-pass filters for oceanic disturbances propagating into the interior of the bays. Overall, tidal variance decreases sharply inside the bays, with the semidiurnal tides suffering significantly higher attenuation than the diurnal tides. At subtidal frequencies, sea level fluctuations in the interior of the bays are primarily forced by coastal sea level fluctuations generated by atmospheric forcing on the adjacent shelf. Local wind forcing within the bays appears to play only a secondary role in modifying the coastally forced sea level. The active subtidal sea level fluctuations induced by the bay-shelf coupling effect experience no appreciable attenuation within the bays. As a consequence, sea level variance at subtidal frequencies is even greater than that at tidal frequencies in one of the inland bays.

Sea-level fluctuations in a coastal lagoon

Sea-level observations made during December, 1979, at six stations in Great South Bay (which is a coastal lagoon on the south shore of Long Island, New York) reveal that there were significant subtidal fluctuations in addition to the tidal oscillations. Harmonic analysis of the tidal oscillations of sea level indicates that M 2 is the dominant tidal constituent. The M 2 amplitude, however, suffered a more than 50% reduction in the interior of the Bay due largely to the narrow inlet. The subtidal sea level fluctuations within the Bay were forced primarily by the low-frequency fluctuations of the adjacent shelf water. The active subtidal exchange induced by this Bay-shelf coupling appeared to have suffered only minor attenuation within the Bay. As a consequence, the variance associated with subtidal sea level fluctuations was greater than that associated with the tidal oscillations over most of Great South Bay.

A simple model of estuarine subtidal fluctuations forced by local and remote wind stress

Observations of estuarine low subtidal sea level and current fluctuations have often shown domination by the remote effects of the wind, acting on the adjacent coastal ocean, over the local surface stress, acting on the estuary itself. The remote effects are transmitted to the estuary by impressment on its mouth of sea level change induced by the onshore component of coastal Ekman transport and become increasingly dominant as the frequency decreases. A simple, barotropic model is developed to investigate the joint action of these two wind forcing mechanisms. The relative shortness of most estuaries relative to low subtidal estuarine wavelengths explains the dominance of the remote effect for both sea level and barotropic current fluctuations in the estuary. For the same reason, however, surface slope is dominated by local wind setup. For an estuary with axis nearly parallel to the coast the two effects will operate either in concert or in opposition, depending on hemisphere and orientatipn of the estuary axis relative to the coast. For the geometries of both Chesapeake Bay and the Delaware Estuary the model predicts opposition with the remote effect dominant at lower frequencies, consistent with recent observations.

Subtidal sea level and current variations in the northern reach of San Francisco Bay

Analyses of sea level and current-meter data using digital filters and a variety of statistical methods show a variety of phenomena related to non-local coastal forcing and local tidal forcing in the northern reach of San Francisco Bay, a partially mixed estuary. Low-frequency variations in sea level are dominated by non-local variations in coastal sea level and also show a smaller influence from tidally induced fortnightly sea level variations. Low-frequency currents demonstrate a gravitational circulation which is modified by changes in tidal-current speed over the spring-neap tidal cycle. Transients in gravitational circulation induce internal oscillations with periods of two to four days.

Low frequency sea level variability in the vicinity of the East River tidal strait

The East River is a narrow tidal strait connecting Long Island Sound with the lower Hudson Estuary; the Sound and Estuary both communicate with the New York Bight. Subtidal fluctuations in sea level difference between the ends of the strait which are coherent with barotropic flow through the strait are produced mainly by direct setup in the Sound and Estuary and by sea level fluctuations (primarily wind forced) in the Bight. Approximately 70% of the variance in subtidal sea level difference can be accounted for by the two components of wind stress. The direction of dominant wind forcing (wind stress direction producing maximum response) varies considerably with frequency. Response functions relating sea level difference to wind stress components computed for 128 day series for sea level and winds allow us to interpret the direction of dominant forcing in terms of the relative contributions of direct setup and sea level fluctuations in the Bight.

Observations of Low-Frequency Variability in Great South Bay and Relations to Atmospheric Forcing

Abstract Sea level and current data collected around Great South Bay, New York during December 1979 are examined in conjunction with atmospheric data for evidence of wind-forced, low frequency variability in the Bay and on the adjacent shelf. The subtidal sea level along the coast was found to be highly coherent from Sandy Hook to Montauk Point, with a single empirical mode accounted for more than 97% of the total variance. These coherent fluctuations were forced primarily by longshore winds (along 250–070°T) through the coastal Ekman effect. The sea level within the Bay exhibited large and spatially coherent subtidal fluctuations as a result of a strong coupling with the adjacent shelf. The characteristic volume exchange associated with this Bay-shelf coupling was an active simultaneous inflow or outflow through both ends of the Bay (with fluctuations in excess of 20 cm s−1) in response to the rise or fall of coastal sea level induced by longshore winds.

Observations of wind‐induced, subtidal variability in the Delaware estuary

Sea level and current observations made in the Delaware estuary during autumn of 1982 were examined for evidence of wind‐forced subtidal variability. The large subtidal sea level fluctuations at the mouth of the Delaware were found to be forced primarily by the wind stress component over the continental shelf nearly parallel to shore, indicating coastal Ekman transport to the right of the wind. Local wind forcing within the estuary is shown to be insignificant. The subtidal sea level in the interior of the estuary was found to be driven by the wind through a combination of two remote forcing mechanisms: one acting at the mouth of the Delaware through direct estuary‐shelf coupling and a second acting locally over the Chesapeake Bay and being transmitted from the upper Chesapeake through the Chesapeake and Delaware Canal, which links the two estuaries. In the upper Delaware estuary we found predominantly barotropic subtidal current fluctuations superimposed on a much weaker two‐layer gravitational circulation. These current fluctuations were produced primarily by the local subtidal surface slope generated by the difference between the two remote forcing mechanisms. The evidence strongly suggests that the Chesapeake and Delaware Canal plays a critical role in the subtidal circulation of the Delaware estuary.

Subtidal Sea Level Variations in the Chesapeake Bay and Relations to Atmospheric Forcing

Abstract Subtidal sea level variations in the Chesapeake Bay were examined over a one-year period for evidence of wind-driven barotropic circulation. The major transport occurred at time scales of 3–5 days, whose magnitude was larger than the river runoff. It was driven by the east-west wind, as part of the coupled coastal ocean-estuary response. At shorter time scales, there was also large barotropic motion which, however, was driven by the local, north-south wind. The variance of barotropic fluctuation was larger by a factor of 4 in winter than in summer, due to the increased cyclone activities. The coupled coastal ocean-estuary response was also more pronounced in winter. In contrast, the summer season was dominated by local forcing at time scales of 3–7 days. The results suggest that the barotropic motion is an important component of the net circulation. The corresponding subtidal sea level change contributes significantly to the storm surge. Thus, the nature of barotropic response, particularly the c...

Subtidal Sea Level Fluctuations and Their Relation to Atmospheric Forcing along the North Carolina Coast

Coastal sea level fluctuations in Onslow Bay are selectively coupled with local atmospheric forcing variables. The coupling is strongest in period bands of 2.5–3.5 and ≳10 days, which span absolute zero group speed, barotropic continental shelf wave periods. The phase of the sea level disturbances propagated upstream from Beaufort to Wilmington, N. C., as expected for stable discrete shelf waves, consistent with earlier results by Mysak and Hamon (1969). The “barometric function”, when corrected for coherent wind stresses, indicated selective, super barometric atmospheric-pressure-to-sea-level coupling in the zero group speed period bands. Stochastic models of atmospheric cold front wind stress and wind stress curl fields were found to selectively force barotropic shelf wave responses near zero group speed periods and wavelengths. A strong Onslow Bay response to the model cold front occurred near the second harmonic forcing frequency, the wind stress curl contributed importantly to the shape and the amplitude of the response spectrum. The results suggest that shelf waves forced by the atmosphere contribute to the Gulf Stream meander field off North Carolina.